The instant invention relates to a novel method and apparatus for determining the relative position of two fixed bodies. More particularly, the instant invention is directed to a method and apparatus for ascertaining the relative position of two underwater pipelines that are to be coupled together.
It has heretofore been found difficult to couple together pipe sections which lay on the floor of a body of water. Often, the topography of the ocean floor, upon which a pipeline is layed, causes a section of pipe to assume an orientation that is different from that of another section. A situation such as this causes the ends of the pipe sections, which are to be joined, to be misaligned.
Additional pipeline misalignment problems may be brought about when it is desired to join two pipelines together which, through laying on a flat ocean floor surface, do not have their centerline axes placed colinearly. Further misalignment problems may be encountered if it is desired to couple a pipeline to a storage tank or the like. The pipeline may approach the storage tank opening at an angle rather than head on presenting yet another type of misalignment problem.
Various devices have been used to effect the coupling of underwater pipeline sections. One such device is that of U.S. Pat. No. 3,658,231, issued to Ocean Systems, Inc. The device disclosed therein includes a frame-like structure having a central chamber and movable end support fixtures. The ends of the pipe sections to be coupled are forced into alignment by pressure actuated mechanisms which engage the pipe sections. Aligning pipe sections by force generally subject the pipe sections to undesirable stresses which can cause serious damage to the pipeline.
Another device for coupling the ends of the pipe sections is that of U.S. Pat. No. 3,393,926. This device utilizes a coupling into which the ends of the pipe sections are received. The ends are gripped and sealed with the coupling by fluid actuated slips within the coupling. This device, though adequate, is not particularly desirable since the coupling must remain on the pipeline resulting in an economic disadvantage. Also, the ends must often be forced into alignment and are thus held coupled together under stress.
Another approach to coupling of the ends of pipe sections is that of Oliver, U.S. Pat. No. 3,835,655. In the technique disclosed, a first marker buoy is used to detachably support a tow line passing from the marker buoy through a guide member mounted on the end of the pipeline and back to the marker buoy. A second tow line connects between an end of another pipeline to be laid and a second marker buoy. The tow lines are connected together and the second pipeling is lowered until adjacent the first pipeline end. The connected tow line is pulled until the pipeline ends are joined. The ends of the second pipeline has a mating section having connections that provide universal movement to permit interconnection of misaligned ends. Although this approach is workable where one pipe end is movable relative to the other, in situations wherein repair of an existing pipeline is to be performed, this approach is not useable.
It is apparent that it would be desirable to be able to couple pipeline sections together by welding or bolting a connecting section of pipe, obviating the need for expensive coupling devices. It is also apparent that it would be desirable to so couple pipe sections without having to force the pipe sections into alignment, thereby reducing damaging stress.
Before a connecting section of pipe can be fabricated, the exact spatial relation of the planar ends of the two underwater pipelines must be determined. One approach in determining the relative position of two submerged pipelines is that of attaching pipe flanges on the ends of the pipelines and welding several connecting rods between the flanges. The flanges are then detached from the pipelines, and the entire structure consisting of the flanges and the connecting rods are brought to the surface. From the orientation of the flanges, the relative positioning of the ends of the submerged pipelines is ascertainable. Further details of this technique may be had by reference to the aritcle "Flanged spool connects subsea pipeline ends", THE OIL AND GAS JOURNAL, Feb. 3, 1975, pp. 92, 93 and 96. A disadvantage of this approach is that the structure of welded rods can be quite heavy, especially if the flanges are a substantial distance apart, and thus difficult to handle. Also, diver time and welding time are significant and make this technique expensive.
Another device for determining the spatial relationship between a pair of displaced pipeline ends is that of Morgan, U.S. Pat. No. 3,667,128. The device disclosed is a gauge having a pair of universally rotatable linkages with flange adapter plates positioned on each end of a support bar. The device is lowered from a lay barge by a winch down to the pipeline ends, and the rotatable linkages are manipulated to bring the flange adapter plates into engagement with the pipeline end flanges. The gauge is then raised to the deck of the lay barge where the exact spatial relationship of the underwater pipe ends may be re-established between a pair of template fixtures. A spool can then be fabricated by conventional means between the templates, and thereafter lowered for installation between the spaced pipeline ends.
Another similar device for determining the spatial relationship of two misaligned ends is that of Fischer et al., U.S. Pat. No. 3,270,426. This device is also an adjustable gauge comprising an elongate connecting member having an end plate at each end that is movable about two pivot axes. The end plates have a face formed thereon that matches the face of the flanges at the pipeline ends. The end plates are fitted into position against the pipe ends and are held by clamping mechanisms. With the end plates so held, the gauge is released from the pipeline ends and taken to the barge where it is used in conjunction with a template to reproduce the relative positions of the pipe ends.
These approaches to measuring the relative positions of two misaligned pipe ends and for reproducing the arrangement present similar disadvantages as the welded rod technique. Primarily, the devices could be quite heavy and cumbersome to manipulate. Also, diver time would likely be significant, making the use of such gauges quite expensive.
Another device for measuring the distance and alignment of flange surfaces so that the relative orientation of two pipe ends can be reproduced is that disclosed in German Pat. No. 128,677. The device comprises a telescoping pipe having a scale marked off over its length for reading the distance to which it is extended. A plate with a protractor is pivoted at each end of the telescoping pipe. One plate pivots about a horizontal axis and the other about a vertical axis. The amount of rotation of the plate is measured by the protractor. With the plates affixed to the faces of the pipeline ends, the distance between them and two angles can be measured. Although the measurements can successfully be used to reproduce the orientation of the pipeline ends by simulation with templates, there is not enough information nor are the measurements so made as to permit an interconnecting pipe spool to be fabricated merely from calculations.